Magnetic head



Dec. 8, 1953 J. R. ANDERSON MAGNETIC HEAD 2 Sheets-Sheet l Filed Feb. 18, 1950 [NVE/V709 J. AAA/05H50 -AJLWM- Arm/mfr I Patented Dec. 8, 1953 MAGNETIC HEAD John R. Anderson, Berkeley Heights, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York,.N. Y., a corporation of New York Application February 18, 1950, Serial No. 145,027

2 Claims. (Cl. 179-1002) This invention relates to magnetic heads for use in magnetic recording, reproduction, or obliteration, and particularly to high frequency heads comprising laminated pole-pieces.

The object of the present invention is to pro`-A vide a high frequency magnetic recording and reproducing head which Will permit the efficient recording of microsecond electrical pulses in the ksmallest possible length of recording medium,

and the reproduction of these pulses without frequency discrimination. Frequency discrimination is here taken to mean that which occurs in A converting electrical current to recording ux or in converting flux in the head when reproducing to integrated open circuit voltage output.

Y In order to record electrical pulses of short duration successfully it is essential that the high frequency eddy' current losses in the recording head be reduced to a minimum to minimize internal magnetic energy loss and thereby provide a strong magnetic energy for the recording medium, and that the magnetic flux reaching the recording medium be concentrated directly under the recording-reproducing gap and reduced to a minimum at all other points. l Y Eddy current losses are directly7 proportional to the square of the thickness of thev individual lamina. Extremely thin laminae are therefore desirable for use in structures working at exceedngly high frequencies where eddy current losses are to be kept at a low value.

In accordance with this invention the abovenoted requirements are satisfactorily met by a novel magnetic head which comprises a delta or triangulareshaped laminated core the laminae of which are not greater than 0.001 inch in thickness and are of a high permeability magnetic material such as the moly-Permalloy comprising 4 per cent molybdenum and 79 rper cent nickel, knife-edged pole tips which approach the recording medium at an angle of approximately 45 degrees and thereby reduce the magnetic flux to a minimum at all points except directly under the recording-reproducing gap,and an eddy current shield of a material such ascopper placed directly v in front and directly behind the pole tips, which further prevents magnetic ux from reaching the recording medium at points other than that directly under the gap.

The nature of the invention and its distinguishing features and advantages will be more clearly understood from the following detailed description and the accompanying drawings in which like reference characters in the different gures designate similar elements and:

Fig. 1 is a front plan view of the assembled magnetic head with part of the front plate cut away;

Fig. 2 is a view in perspective with the front plate removed showing the arrangement of the pole-pieces, coils, and eddy current shield;I

Fig. 3 is a functional sketch showing the recording head and the related recording medium; and

Fig. 4 shows a comparison of the frequency response of each of two delta heads employing specinc winding systems with the response of a comrnercial ring-type head designed for audio frequency operation.

In' one embodiment of the invention, which is shown in Figs. 1 and 2, the triangular-shaped core comprises three laminated pole-pieces 2, 4 and 5 which comprise laminae of high permeability magnetic material having a thickness not greater than 0.001 inch. Pole-pieces 2 and 6 converge inwardly to provide a gap 28 and mounting means for the recording, reproducing and obliterating coils. This type of construction permits fabrication of the pole-pieces by cutting apart square or triangular cores of high permeability magnetic tape prepared in accordance with the process described in patent application Serial No. 565,890, filed November 30, 1944, by K. G. Compton and H. G. B. Gould. A practical method found for cutting the laminated core without splitting apart or damaging the laminae comprises the steps of casting the core in a casting resin, cutting the pole-pieces from said core and lappingthem while in the casting resin, and then manually separating the cut pole-pieces from the casting resin. The casting resin acts as a disposable clamp for holding and backing up the magnetic material at all points during the cutting and lapping operations.

The arrangement of windings on pole-pieces 2 and 6 for recording and reproducing depends, of course, upon the electronic equipment to be used with the magnetic head. In the particular embodiment of the invention shown in Figs. 1 and 2 each of the winding systems 8 and l0 on polepieces 2 and B, respectively, comprises a single layer winding. These windings may be connected series aiding, and recording and obliterating may be done with current through one or both of the windings. Both windings are used for reproducing. This arrangement appears to be most suitable where the reproducing process may be carried out after all recording has been completed.

As shown in Fig. 3, each of the winding systems S and l0 may be a double-layer system comprising a reproducing coil I2 having a relatively large number of turns, a recording coil I4 and an obliterating coil I6, each containing a small number of turns. The reproducing coils are connected series aiding. This type of arrangement may be preferred where large signals are required in reproduction and microsecond pulses are to be recorded.

Fig. 4 shows the frequency response of uX in the recording-reproducing gap produced by current into the recording-reproducing coils of a typical commercial ring-type head, and delta :i heads with winding systems such as described above. 1 Curyes.v A; and B were obtained.fwith a deltarhead employing a doublewindingsystem such as shown in Fig. 3 wherein 480 turns were used for reproducing, 240 turns being wound around each of pole-pieces 2 and 6, and-24turns were used for recording. The 24 turns for recording were wound around poleg-piecefhwliile 24 turns for obliterating Werewound aroundpolepiece 6. Curve C was obtained'withadelta head* employing a single layer Winding system such as shown in Figs. 1 and 2 whereri1120fturnsfwere used for both reproducing and recording, .60 turns being wound around each of lpole-pieces 2 and 6. The use of single-layer coils having low f-distribu-tedl capacitanceandi low capacitance to ground substantially --improves the `frequencyresponse. As can be'seen in Fig. 4, the coilfreso- :nance peak at `one megacycle ywith 480 turns "in thereproducing coils was eliminated by-'using two f60-turn coils. It has-been! found" that, -in :addition tof its wide frequency fresponse, at recording head in accordance witlr the-presentinvention' isc capablel of recordingshort lpulses in one-half the space required yfor commercial ringtype -heads=with an increase -in eciencyof-about -iiifty times.

As' shown in Figs. liand2 thepole-piecesare securely positioned within the guide platesE I8 V--byimeansof aretaining-spring'Z, the back plate i22-and thefront plate-'24. Thegguide plates I8 Vare-made `of Ia shielding -material suchA as 'l copper and are the eddy currentY shieldspositioned` -directly inf front and directlybehind the polef tips 21# and 29- to preventmagnetie4 flux fromreach- *ing-fthe Arecordingmedium 26 (see-Fig."-S) at Apointsother thanY that Adirectly under the recording-reproducing -gap 428. The retaining spring '2U-may be madefrom; a material such as `Phosphor 1'bronZe,*-while the 1back plate22 and front vplate 24 are preferably f made -from a vnon-magnetic material such as -nickel4 silver. The recording headmay be assembledsatisfactorilyein the following manner. ATheguideygilates-I8 are-affixed Ato the backA plate 22'by; soldering. incarrying out this step a;jig maybe; employed inthe conventional manner' to assure'` 4that vthe supporting surfaces forpole-pieces `2 and 6 will converge at the speciedangle and thatI the-gap atgthe convergingA point willV be, suiciently]large to permit the knife.k edges .of the pole tips to project` beyond M.the eddy. current shield elementsj -as shown in"1iigs.,1.l and 2. The vpole-pieces arethen placed A.Within the guides lhandv` held in position with ..the,cooperation ofthe.A retaining 'spring 2 0,coi1s l.Rand i havingV previously been positioned about r.pole-pieces 2 and, respectively,v and having been securely cemented thereto, ,and aw shimyhaving `been -positioned betweenv the inwardly`vcenvergjingl pole tips'2'l andk 29 orpole-pieces 2f and6 to G0 `,provide aV gap of substantially 0'.0Q2inch. The

liront plate 24 is then secured to :the baclgplate 22 by means of screws 30 ,thereby clamping `the pole- ,pieces injthe position I determined; by the guide nchshim. placedL between, the pole., ,tipsA of `1201e- .piec.es,.2 and 6. ..Thes,hm.Sthen removed.

.Recordingheads. in4 accordance with. .the present invention havebeen operatedsuccessfullyin ccmbination with a .recording medium 26A comprising a cobalt-nickel plated brass cylinder havingt a 4 thick. The Asystem has ,been operated with the f polezitipsr` out of contaptwith thefrecording me- 'dium, a gap of approXimately'ODZ' inch being maintained between the pole tips and the mez`5 dium, and with the cylinder revolving at a speed :0517-70 revolutions per minute.

It is to be understood that the above-described f arrangements are' illustrative of the application `DLtheiprinciples of the invention. Other arlcrangementmaylbe devised by those skilled in the art without departing from the spirit and 'afscope f-` thefinvention.

Whatis claimed is: lxAnelectromagnetic transducer comprising two pole-pieces and a yoke, said pole-pieces and Y yoke being-- of :substantially the same thickness :-andwidthand 'comprisingan=equ'a1-number: of laminations of Lmagnetic 'materialthaving-a Ithickness of substantiallyIOOl mch,-said=lamlna'tions of said pole-pieces being trapezoidallyfshaped :saidl lamina-tions of2 said. yoke being rectangular- 1 ly shaped.l adherence Aproducing and'insulatlng `separators between said Jlarninations,L said-ecp- Y arators forminga unioni between adj acentfflaminations, said pole-pieces and ryoke 'being fdis- --posed in such manner as -tov 'form' a; stack-0fy substantially'identical triangularly shaped magnetic cores, said ypole-pieces comprising lknifeee'dged Mpole vtipsy and converging to-form a gap," each 30 of saidi-lamina-tions infsaid-yokecontacting a f corresponding -laminationn'each of-said-pole pieces, a non-magnetic -eddycurrent shield, said pole-pieces disposed-withinsaid shield' Vsuch #manner that substantially' only the-knife edges; of said pole tips project -fromsaidshield.

y 2. electromagnetic --transducerin accordance withclaim 1u wherein a coilffor reproducing; signalsA is disposedaround each lof said polepieces, y said coils --comprise lthe same Y relatively 4f) largenumber of turns and are connected in series aiding relation, a coil i for recordingA signals, is disposed around one ofMsaidpolep iece sa vcoil for -obliterating signals is 'disposed ,aroundthe other of `saidpole-pieces Mand each of Vsaid recording'and obliterating coils comprises a relay tively smallA number o fv turns.

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